In a primary/secondary configuration, the heating zones normally come off the secondary loop. The primary loop may or may not come on to provide heat to them, and may be controlled by the high/low boiler controls - the primary loop would only come on when the boiler was on (and may remain on to purge the heat exchanger for a bit), while the secondary loop would run whenever a thermostat called for space heating (they may not both always be on).
But, when the DHW calls for heat, assuming it is on a priority zone, you'd want to fire the boiler (which would cause the primary loop pump to turn on) and turn off the secondary loop to allow all of the heat to get to the DHW. the goal is to reheat the DHW as fast as you can, whereas the space heating may not want or need that high of a fire rate, and the heat in the secondary loop may be sufficient. To ensure you get max heat to DHW, you turn on the primary, and off the secondary, then max heat is available to DHW production.